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Flows at these locations are used to estimate the total annual delivery obligation, to determine <br />deliveries to date, and to establish a "curtailment" of water use if needed to meet the delivery <br />obligation of the remainder of the year. These calculations are made by the Division Engineer <br />and his staff every 10 days when diversions are being made, and monthly for the remainder of the <br />year. Calculations and accounting occur independently for both the Rio Grande and Conejos <br />River. <br />The general methodology for making these calculations is described by four steps. The actual 10- <br />day Compact analysis worksheets used by Division 3 are provided as Attachment 3 (the example <br />sheet shown is for conditions at the end of June 1998). A 10-day report showing monthly <br />accumulated deliveries and curtailmentpercentages is also developed and an example (1998) is <br />provided as Attachment 4. As described, the period over which calculations are made every 10 <br />days can vary from year to year depending on when diversions begin and end, but these 10-day <br />calculates typically occur over the period April 1St through October 31St <br />January 1St through March 31St <br />Both the Rio Grande and the Conejos River are typically curtailed 100 percent, <br />i.e. no diversions are allowed except for storage by pre-Compact reservoirs. <br />Exceptions might occur if Colorado is in a significant credit situation or if <br />demand is very high due to drought conditions. <br />April 1St through October 31St -every 10 days <br />Step 1. Estimate Index Supply for the Year <br />Deliveries to date (Jan. -Mar.) at the Index gages are added to forecasted inflows <br />from NRCS (Apr. -Sept.) and historical averages for Oct. -Dec. This provides <br />a first estimate of the total annual Index Supply on April 1. For every estimate <br />thereafter, the actual flows occurring since April are subtracted from the Apr. - <br />Sept. forecast. <br />Step 2. Determine Obligation (Annual Delivery Requirement) <br />The annual obligation (delivery requirement) is determined from the <br />tables/percentages found in Article III of the Compact. <br />Step 3. Estimate Deliveries for the Year <br />Deliveries to date are added to estimates of annual deliveries from the Closed <br />Basin Project and the annual Paper Credit. For the Rio Grande, the estimated <br />annual flow of the South Channel of the Norton Drain is subtracted because this <br />Conejos Basin credit is included in the gaged flow of the Rio Grande near <br />Lobatos. For the Conejos River, the estimated annual flow of the South channel <br />of the Norton Drain is added to the deliveries at La Sauces. <br />The "needed" flow between the current date and the end of October required to <br />reach the obligation (delivery requirement) estimated in Step 2 is then calculated. <br />Step 4. Calculate Curtailment Percentage <br />The curtailment percentage is determined by dividing the result from Step 3 <br />(needed flow between the current date and the end of October) by the estimated <br />C:\cdss\Div3_Mem2.doc Division 3 Interview June 28, 1999 -Page 6 of 8 <br />